System and method for purifying exhaust gases

ABSTRACT

A system for purifying exhaust gases from a diesel or like engine ( 1 ) comprises a filter ( 3 ) and a plasma generator ( 5 ). The plasma generator converts NO and/or N 2  to NO 2  and/or generates ozone, which is found to be particularly effective at causing low temperature combustion of soot trapped on the filter.

This application is the U.S. national phase application of PCTInternational Application No. PCT/GB99/03102.

This invention concerns the purification of exhaust gases, especiallythe purification of such gases from diesel and other “lean-burn”engines.

Diesel engines are in widespread use in all types of vehicles,stationary power sources and naval and commercial shipping. They arevery fuel-efficient, but because of their combustion characteristicsgenerate particulate matter (soot, often called “PM”) on which a varietyof organic substances may be absorbed, including unburnt hydrocarbons(HC) and sulphuric acid produced by oxidation of sulphur dioxide derivedfrom sulphur species present in the fuel or in lubricants. Otherengines, such as gasoline direct injection (“GDI”), can also producesignificant quantities of PM, and we consider that the need for removingsuch PM will soon be expressed in legislation. Nonetheless, theinvention may be applied to combustion processes generally, as well aspotentially to chemical process stacks/exhausts, and to combustionengines operating at λ=1 or greater, or lean-bum engines operating atstoichiometric or rich in order to regenerate some exhaust gasaftertreatment device. For simplicity, however, we concentrate on dieselengines hereinafter.

In order to meet various regulations concerning the level of pollutants,it has become commonplace to fit vehicles with an oxidation or three-waycatalyst, which only achieves partial removal of PM. The removal ofparticulates is generally achieved by using some form of filter or trap,which may be cleaned or regenerated intermittently. It has beensuggested to include a catalyst in the fuel to the engine, and as wellas platinum group metals (“PGMs”), iron, copper or cerium compounds havebeen suggested. A particulate trap may be catalysed to lower the sootcombustion temperature, and some form of external heating, for exampleelectric heating of the trap or of air fed thereto, may be used toinitiate soot combustion.

A particularly successful soot trap is marketed by Johnson Matthey PLCas the “CRT” (“Continuously Regenerating Technology”) and is describedin U.S. Pat. No. 4,902,487. This system uses a conversion of NO in theexhaust gas to NO₂, which was discovered to be much more effective attypical low diesel exhaust gas temperatures in the combustion of sootthan air or any other exhaust gas component. Thus, NO₂ is typicallyeffective to combust PM at about 250° C., whereas oxygen is effective atabout 650° C.

It has been suggested to use a plasma generator for exhaust gaspurification (see for example GB 2,274,412 and 2 270 013, UK AtomicEnergy Authority). Although it was probably not previously recognised inconnection with exhaust gas treatment, such a system producesconsiderable quantities of NO₂. Systems such as previously described donot include any filter or trap in combination with a plasma generator,but we believe that this may be a particularly effective system fortreating diesel and similar lean-bum exhaust gases.

Accordingly, the present invention provides a system for the treatmentof such exhaust gases, comprising a plasma generator effective toconvert at least a portion of NO and/or nitrogen in the exhaust gases toNO₂ and/or to generate ozone and a filter effective to trap a desiredproportion of soot from the exhaust gases, whereby said trapped soot iscombusted by reaction with NO₂ and/or ozone, at a markedly lowertemperature than required by O₂.

We believe, although we do not wish to be bound by any theory, that inthe present invention NO₂ may be generated not only by oxidation of NOin the exhaust gases, but also by oxidation of nitrogen to yield NO,which is itself converted to NO₂. In the latter case, there is noreliance upon the quantities of NOx leaving the engine. It is alsobelieved that the present invention is especially valuable in that it isnot adversely affected by the presence of sulphur in the fuel or inlubricants, which can poison conventional catalysts.

The invention further provides a method of reducing emissions fromexhaust gases from diesel and like engines, comprising trapping soot ona filter and continuously or intermittently combusting the soot byreaction with NO₂ and/or ozone produced by a plasma generator,preferably using a plasma treatment of at least a portion of the exhaustgases.

The plasma generator may be any suitable type producing a non-thermalplasma, and may be enhanced by electromagnetic radiation. Suitableplasma generators include high voltage (e.g. 20 kV or more) alternatingcurrent, preferably pulsed, generators, suitably using two dielectricplates positioned in the gas flow, and piezoelectric devices such aspiezoceramic transformers. It may be positioned to treat all or aportion of the exhaust gases upstream of the filter, or may be fitteddownstream of the filter to treat all or a portion of the filteredexhaust gas, with recirculation of plasma-treated gases to the filter.In one embodiment of the present invention, a pre-determined proportionof the exhaust gases is treated by the plasma to cause substantially allof the NO present to be converted into NO₂, and the resulting gasesblended with untreated exhaust gases, thus resulting in a blend of NOand NO₂, which according to some studies, may be more effective for thepurposes of the present invention than a gas containing substantiallyonly NO₂ in admixture with other exhaust gas components.

The filter used may be a woven or knitted wire filter, a gas-permeablemetal or ceramic foamed mass or a wall flow filter of generally knowntype (honeycomb monolith). For certain vehicles, especially light carsor vans, it may be necessary or desirable, to use a filter design whichcollects only 80% or so by weight of the total soot particulates andpreferably incorporates a by-pass and/or pressure relief valve. Thefilter may be partially or completely catalysed if desired. A catalysedtrap may improve the aggregate removal of pollutants.

A modification of the present invention incorporates a means forremoving NOx downstream of the filter and plasma generator. Such meansmay be a NOx trap, which technology is available to the skilled person,and generally includes one or more alkali earth metal compounds,especially calcium oxide or barium oxide, or alkali metal, carried on ametal or ceramic honeycomb-type support. The NOx trap is desirably usedin combination with a lean-NOx catalyst. Another means for removing NOxis Selective Catalytic Reduction (“SCR”), which is well established forstationary power sources and is receiving increasing attention forvehicular applications. Such a modified system can be effective to meetall current and known future emission control regulations for diesel andlike engines.

The plasma generator may be controlled and actuated by an enginemanagement unit, or other microprocessor control unit, to operateintermittently according to certain engine operating conditions (speed,load etc) which have been pre-determined to generate more soot. Theidentification of such operating conditions can be readily accomplishedby one skilled in the art, and such conditions include times when theemission of particulate matter would be undesirably high in the absenceof any treatment therefor. Alternatively, the plasma generator mayoperate during all operational conditions of the engine, which systemhas the benefit of simplicity, but this may be undesirable if the engineis in an operating condition in which significant quantities of NOx aregenerated, or during regeneration of a NOx trap.

The present invention, at least in its most preferred embodiments, inaddition to being particularly effective at controlling emissions,permits the engine designers to design and tune the engine for powerand/or fuel efficiency, rather than being forced to make compromises inengine design to minimise the generation of NOx and particulates. Thiscan be a significant advantage for commercial vehicles, but allowsflexibility in design for all engines and types of vehicles.

A further variant of the present invention is to feed a reductant, whichterm includes hydrocarbon fuel, e.g. diesel fuel, ammonia, ammoniaprecursors, hydrogen etc. into the exhaust gases either upstream ordownstream of the plasma generator.

The present invention is illustrated with reference to the accompanyingdrawing, which is a schematic diagram of a system according to theinvention.

A diesel engine is shown at 1, and has an exhaust system, 2.Conventional silencer boxes and ancilliary equipment are not shown. Awall flow filter, 3, retained within a metal box, 4, is mounted in theexhaust system. Mounted close upstream to the filter, is a plasmagenerator, 5, which is operated according to signals from the enginemanagement unit, 6.

Testing of the described system is continuing, but early indications arethat substantially all soot particles trapped on the filter are removedcontinuously, although there are variations in soot build up and removalrates. NO₂ and ozone have been detected in the exhaust gases after theplasma generator, with substantially lower levels after the filter.

The following Examples illustrate features of the present invention.

EXAMPLE 1

The non-thermal plasma discharge generator used comprised a ceramic tube10 cm long and 5 cm external diameter in which a bed of pelletedmaterial of suitable dielectric constant was held between two circularstainless steel mesh electrodes. The mesh aperture size was about 0.5mm. Typically the pellets were of size about 3 mm, and occupied a lengthof 1-3 cm in the ceramic tube. The packed volume was about 12-36 cm³.One electrode was grounded via a large spring that maintained a physicalpressure on the bed of pellets. The other electrode was fixed andconnected to the ‘live’ side of a power supply capable of providing anadjustable AC voltage of 50 Hz up to 10 kV and powers of up to 1 kW.

A gas mixture designed to approximate key features of exhaust gas from adiesel engine containing nitric oxide (300 ppm), propene (300 ppm),oxygen (12%), and water vapour (about 1%) with the balance being heliumwas passed through the plasma generator at a flow rate of 250 ml min⁻¹.A mass spectrometer was used to determine and quantify the compositionof gas exiting the generator. When operating at ambient temperature witha voltage of about 3 kV applied across the electrodes destruction ofpropene was almost 100%, and a large quantity of carbon dioxide wasformed. However, the amount of carbon dioxide was only about 35% of thatexpected for complete combustion. Traces of formaldehyde were detectedbut carbon monoxide probably accounted for most of the other oxidationproducts. However, its quantification was complicated by traces ofnitrogen having a similar mass number.

Nitric oxide was also completely removed when the potential was appliedto the electrodes, and substantial levels of nitrogen dioxide (mass 46)were detected in the exit gas. The amount of nitrogen dioxide detectedtypically corresponded to about 55% of the amount of the original nitricoxide and depended on the nature of the pellets. With alumina pelletshigher surface area material (eg 200 m² g⁻¹) gave higher conversionsthan low surface area material (eg 5 m² g⁻¹). Alumina pellets coatedwith a thin layer of barium titanate or lead titanate gave higherconversions than just pure alumina pellets. Increasing the voltageapplied across the electrodes also increased conversion of nitric oxideto nitrogen dioxide. These experiments demonstrate nitric oxide isoxidised to nitrogen dioxide by passage through a non-thermal plasmaeven when hydrocarbon is present.

EXAMPLE 2

A cordierite wallflow filter (5.66 inch diameter, 6.0 inch long) having100 cells inch⁻² and {fraction (17/1000)} inch thick walls was locatedin the exhaust pipe of a four cylinder 1.9 litre direct injection turbocharged Diesel engine that ran on fuel containing 350 ppm sulphur. Theengine was operated at 1200 rpm with half load for 10 hours. The filterwas then removed from the exhaust pipe and ground to a powder that waspressed into small granules (250-350 μm). A sample of these sooty blackgranules (0.05 g) was placed in a stainless steel tube (6 mm diameter)and held in place by two small loose plugs of quartz wool. The tube wasconnected to the exit of the plasma generator of Example 1, and the gaswas heated to temperatures in the range of 150-300° C. by electricalheating tape before passing over the sample containing Diesel soot.Analysis of the gas once it had passed over the soot containing samplewas achieved by a mass spectrometer. Increasing the temperature of thegas passing over the sample resulted in increasing amounts of carbondioxide being formed, and increasing amounts of nitric oxide in the gasafter the sample. After maintaining the sample temperature at about 240°C. for an hour the discharged granules had only a light grey colourationindicating that most of the soot had been removed by exposure to theplasma treated gas. This experiment shows that gas containing nitricoxide that has been oxidised in a non-thermal plasma generator oxidisesDiesel soot at temperatures above about 150° C., and so such a devicecould be used to keep a Diesel particulate filter free of excess soot bycontinuously combusting soot, even at relatively low temperatures.

It will be appreciated that many variations may be made to the system asparticularly described, without departing from the present inventiveconcept. In particular, the skilled person will appreciate that Example2 above illustrates a practical method for removing soot from a filtereven at the low temperatures met with many modern engine designs,particularly when operating at idle or under low load. This is avaluable contribution to the art.

What is claimed is:
 1. A system for treating exhaust gases including NO,nitrogen and particulate matter, which system comprising a plasmagenerator for generating at least one of: (1) NO₂ from the NO ornitrogen or both; and (2) ozone, and a filter for trapping a desiredproportion of the particulate matter and a microprocessor forcontrolling the plasma generator to operate during engine conditionswhich generate increased amounts of particulate matter, wherein trappedparticulate matter is combusted by reaction with at least one of NO₂ orozone, and wherein the plasma generator is located downstream of thefilter and all or a portion of the plasma-treated and filtered exhaustgases are recirculated to the upstream side of the filter.
 2. A systemaccording to claim 1, further comprising means for removing or reducingNOx mounted downstream of the filter and the plasma generator.
 3. Asystem according to claim 2, wherein the means for removing or reducingNOx comprises a NOx trap.
 4. A system according to claim 2, wherein themeans for removing or reducing NOx comprises SCR.
 5. A system accordingto claim 1, wherein the plasma generator comprises a piezoelectricdevice.
 6. A system according to claim 1, wherein the microprocessor iscomprised in an engine management unit.
 7. A system according to claim1, wherein a proportion of the exhaust gases are plasma treated andblended with untreated exhaust gases to create a desired blend of NO andNO₂.
 8. A system according to claim 7, wherein all the plasma-treatedand filtered exhaust gases are recirculated to the upstream side of thefilter.
 9. A system according to claim 1, wherein the portion of theplasma-treated and filtered exhaust gases are recirculated to theupstream side of the filter.
 10. A method of reducing exhaust gasemissions from diesel engines, comprising the steps of: creating adesired blend of NO and NO₂ by plasma treating a proportion of theexhaust gases to generate NO₂ during engine conditions which generateincreased amounts of particulate matter and blending the plasma-treatedexhaust gases with untreated exhaust gases, wherein the proportion ofexhaust gases is selected to achieve the desired blend and plasmatreating the proportion of exhaust gases further generates ozone;trapping particulate matter on a filter; and intermittently combustingthe trapped particulate matter by reaction with NO₂ in the desired blendand with ozone.
 11. A method according to claim 10, wherein all, or aportion of, the exhaust gases are passed through the plasma generatorand contacted with trapped particulate matter.
 12. A system for treatingexhaust gases from diesel engines including NO, nitrogen and particulatematter, which system comprising a plasma generator for generating atleast one of: (1) NO₂ from the NO or nitrogen or both; and (2) ozone,and a filter for trapping a desired proportion of the particulate matterand a microprocessor for controlling the plasma generator to operateduring engine conditions which generate increased amounts of particulatematter, wherein trapped particulate matter is combusted by reaction withat least one of NO₂ or ozone, and wherein the plasma generator islocated downstream of the filter and all or a portion of theplasma-treated and filtered exhaust gases are recirculated to theupstream side of the filter.
 13. A system according to claim 12, furthercomprising means for removing or reducing NOx mounted downstream of thefilter and the plasma generator.
 14. A system according to claim 13,wherein the means for removing or reducing NOx comprises a NOx trap. 15.A system according to claim 13, wherein the means for removing orreducing NOx comprises SCR.
 16. A system according to claim 12, whereinthe plasma generator comprises a piezoelectric device.
 17. A systemaccording to claim 12, wherein the microprocessor is comprised in anengine management unit.
 18. A system according to claim 12, wherein aproportion of the exhaust gases are plasma treated and blended withuntreated exhaust gases to create a desired blend of NO and NO₂.
 19. Amethod according to claim 18, wherein all the plasma-treated andfiltered exhaust gases are recirculated to the upstream side of thefilter.
 20. A method according to claim 12, wherein the portion of theplasma-treated and filtered exhaust gases are recirculated to theupstream side of the filter.
 21. A method of reducing exhaust gasemissions from diesel engines comprising trapping particulate matter ona filter and intermittently combusting the trapped particulate matter byreaction with at least one of NO₂ or ozone generated by a plasmagenerator during engine conditions which generate increased amounts ofparticulate matter, and wherein the plasma generator is locateddownstream of the filter and the method further comprises recirculatingall or a portion of the plasma-treated and filtered exhaust gases to theupstream side of the filter.